We note that, in general, for loop calculations some care is needed
when using dimensional regularization. To take advantage of the
spinor-helicity formulation in a one-loop calculation we need to
choose an appropriate version of dimensional
regularization. Specifically, instead of taking the external
polarization tensors and momenta to be (\(4-2\epsilon \) )-dimensional as
in conventional dimensional regularization [1]}, we use the
so called four-dimensional helicity (FDH) scheme [2]}, [3]} where both external and loop state counts are kept in
four dimensions and only the loop momentum is continued to
\(4-2\epsilon \)  dimensions. Because the massive one-loop amplitudes
that we obtain here are neither ultraviolet nor infrared divergent,
the precise distinction between the different versions of dimensional
regularization drops out from the final results for the amplitudes.
We do, however, need to regularize intermediate steps because
individual loop integrals are ultraviolet divergent, with the
divergence canceling in final results.
